Two-photon interference at telecom wavelengths for time-bin-encoded single photons from quantum-dot spin qubits

Quantum physics can empower current network infrastructures with fundamentally new functionalities¹ such as quantum key distribution, distributed quantum computation, quantum clock synchronization and very-long-baseline interferometry, motivating the research for a quantum internet². Essential to the quantum internet is establishing entanglement between remote quantum nodes, which is practically realized by extending spin-photon entanglement^{3, 4, 5, 6, 7, 8} using indistinguishable photons⁹ in Bell state measurements^{10, 11, 12, 13}. Once entanglement is established, information can be transferred between the nodes using quantum teleportation. However, in a quantum internet that comprises multifarious nodes and spans long distances, such a protocol may fail because of photon attenuation, which-path information leakage, decoherence and distinguishability. First, photon attenuation over optical fibres worsens at the native wavelengths of quantum nodes with demonstrated quantum computing capability¹⁴. The attenuation exponentially adds up to more than 30 dB at a node spacing of 10 km, as is commonly assumed in quantum repeater research¹. Second, photons that are entangled with spins may leak which-path information associated with the energy difference between nondegenerate spin states. The eventual registration of photons then causes the spin states to mix incoherently. Third, photon polarization in standard optical fibres changes uncontrollably owing to birefringence, which is unavoidable in practice because of stress and temperature variations and renders the common polarization encoding of photonic qubits^{3, 4, 5, 6, 7} prone to decoherence^{15, 16}. Converting orthogonally polarized photons with equal efficiencies would also require nontrivial engineering of frequency-conversion devices that are generally polarization dependent. Fourth, photon distinguishability due to the heterogeneity among quantum nodes hinders two-photon interference that is crucial to the remote establishment of entanglement; this problem has motivated research on both intrinsic strain¹⁷ and electric field¹⁸ tuning as well as extrinsic quantum frequency upconversion^{19, 20}. However, none of these techniques can be generally applied to heterogeneous quantum nodes while simultaneously achieving long communication distances.